A Faraday rotator is a device formed by a magnetic circuit for a Faraday rotator and a Faraday element, passing light only in one direction by a Faraday effect and blocking the same in the opposite direction. The Faraday rotator is so formed that the plane of polarization of a laser beam reaches a prescribed rotation angle when the laser beam is emitted from the Faraday element, if a magnetic field is applied to the Faraday element with the magnetic circuit for a Faraday rotator.
The Faraday rotator is applied to various uses, and a Faraday element of rare iron garnet such as yttrium iron garnet (YIG) is used as a Faraday element in a Faraday rotator for communication. A ferrite magnet is used for a magnetic circuit for a Faraday rotator creating a magnetic field applied to the Faraday element.
In a case of employing a Faraday element of rare earth iron garnet such as yttrium iron garnet (YIG) for a Faraday rotator for a high-output laser used for working or marking, on the other hand, a crystal of the Faraday element so absorbs light that the temperature rises. As a result, there has been such a problem that the laser beam goes out of focus to exert an influence on light blocking properties of the Faraday element. Therefore, a crystal of terbium gallium garnet (TGG) having small temperature dependence (hardly causing out-focusing resulting from temperature rise) is employed for a Faraday rotator for a high-output laser as a Faraday element.
However, this TGG has a small Faraday rotation factor (Verdet constant) as compared with rare earth iron garnet such as yttrium iron garnet (YIG). In order to obtain a prescribed rotation angle, therefore, it has been necessary to improve the strength of the magnetic field applied to the Faraday element or to lengthen the Faraday element. In the case of lengthening the Faraday element, there has been such an inconvenience that the magnetic circuit for a Faraday rotator in which the Faraday element is arranged also lengthens and the size of the Faraday rotator increases. Further, there has also been such an inconvenience that light is distorted in the crystal if the crystal of TGG itself serving as the Faraday element is formed long and hence high-priced optical glass for correction is also required. Therefore, a magnetic circuit for a Faraday rotator for preventing the Faraday rotator from increase in size is known in general. Such a magnetic circuit for a Faraday rotator is disclosed in Japanese Patent Laying-Open No. 2009-229802, for example.
In Japanese Patent Laving-Open No. 2009-229802, there is disclosed a miniature Faraday rotator including a magnetic circuit constituted of a first magnet magnetized in a direction perpendicular to an optical axis and directed toward the optical axis, a second magnet magnetized in a direction perpendicular to the optical axis and separating from the optical axis and a third magnet, arranged therebetween, magnetized in a direction parallel to the optical axis and directed from the second magnet toward the first magnet and a Faraday element. The magnetic circuit of this miniature Faraday rotator according to Japanese Patent Laying-Open No. 2009-229802 is provided with a hole portion in which the Faraday element is arranged. The miniature Faraday rotator is so formed that the direction of a magnetic field constituted of the first magnet and the second magnet in the hole portion is a direction parallel to the optical axis and directed from the first magnet toward the second magnet. In other words, the miniature Faraday rotator is so formed that the direction of the magnetic field constituted of the first magnet and the second magnet in the hole portion is a direction opposite to the direction of magnetization of the third magnet. Further, the miniature Faraday rotator is so formed that the relation of L2/10≈L3≦L2 holds assuming that L2 represents the length of the first magnet and the second magnet in the optical axis direction and L3 represents the length of the third magnet in the optical axis direction.